1. Field of the Invention
This invention is in the field of freeform fabrication and is directed to the fabrication of three-dimensional objects by selective laser sintering. More specifically, it is related to temperature control in the process chamber of a laser sintering system.
2. Description of the Relevant Art
The field of freeform fabrication of parts has, in recent years, made significant improvements in providing high strength, high density parts for use in the design and pilot production of many useful articles. Freeform fabrication generally refers to the manufacture of articles directly from computer-aided-design (CAD) databases in an automated fashion, rather than by conventional machining of prototype articles according to engineering drawings. As a result, the time required to produce prototype parts from engineering designs has been reduced from several weeks to a matter of a few hours.
By way of background, an example of a freeform fabrication technology is the selective laser sintering process practiced in systems available from 3D Systems, Inc., in which articles are produced from a laser-fusible powder in layerwise fashion. According to this process, a thin layer of powder is dispensed and then fused, melted, or sintered, by laser energy that is directed to those portions of the powder corresponding to a cross-section of the article. Conventional selective laser sintering systems, such as the Vanguard system available from 3D Systems, Inc., position the laser beam by way of galvanometer-driven mirrors that deflect the laser beam. The deflection of the laser beam is controlled, in combination with modulation of the laser itself, to direct laser energy to those locations of the fusible powder layer corresponding to the cross-section of the article to be formed in that layer. The computer based control system is programmed with information indicative of the desired boundaries of a plurality of cross sections of the part to be produced. The laser may be scanned across the powder in raster fashion, with modulation of the laser affected in combination therewith, or the laser may be directed in vector fashion. In some applications, cross-sections of articles are formed in a powder layer by fusing powder along the outline of the cross-section in vector fashion either before or after a raster scan that xe2x80x9cfillsxe2x80x9d the area within the vector-drawn outline. In any case, after the selective fusing of powder in a given layer, an additional layer of powder is then dispensed, and the process repeated, with fused portions of later layers fusing to fused portions of previous layers (as appropriate for the article), until the article is complete.
Detailed description of the selective laser sintering technology may be found in U.S. Pat. Nos. 4,863,538, 5,132,143, and 4,944,817, all assigned to Board of Regents, The University of Texas System, and in U.S. Pat. No. 4,247,508, Housholder, all incorporated herein by this reference.
The selective laser sintering technology has enabled the direct manufacture of three-dimensional articles of high resolution and dimensional accuracy from a variety of materials including polystyrene, some nylons, other plastics, and composite materials such as polymer coated metals and ceramics. Polystyrene parts may be used in the generation of tooling by way of the well-known xe2x80x9clost waxxe2x80x9d process. In addition, selective laser sintering may be used for the direct fabrication of molds from a CAD database representation of the object to be molded in the fabricated molds; in this case, computer operations will xe2x80x9cinvertxe2x80x9d the CAD database representation of the object to be formed, to directly form the negative molds from the powder.
Current commercial laser sintering systems, such as those sold by 3D Systems Systems, Inc. of Valencia, Calif., utilize dual piston cartridge feed systems with a counter-rotating roller and an infrared sensor or pyrometer to measure the thermal conditions in the process chamber and the powder bed.
Although laser systems have proven to be very effective in delivering both powder and thermal energy in a precise and efficient way the use of a single infrared sensor focused on one point on the target surface has some known limitations. The target surface does not normally have a uniform temperature across the entire surface. Thermal gradients are possible from front to back of the process chamber and powder bed due to the presence of an observation window at the front of the system. Gradients are possible from side to side due to the presence of lower temperatures at each side of the part bed. In addition, the recently fused part in the system is hotter than the surrounding powder. Recognizing this, other investigators have proposed other approaches to temperature control in laser sintering.
U.S. Pat. Nos. 5,427,733, 5,530,221, 5,393,482, and 5,508,489, all by Benda et. al. and assigned to United Technologies address this issue with approaches based on an optics and scanning system that detects the temperature of the powder at a detection point near the sintering location and uses that information to modify the laser power and/or modify the temperature of the surrounding powder by use of a traveling defocused laser beam. In this approach and others similar to it, the control is achieved by control of the laser beam power and not by control of a radiant heater. This approach has not seen widespread commercial implementation, probably due to the required sophistication and expense of the optics system as well as issues around quality of the radiated temperature signal from the powder as different powders are employed.
A different approach was proposed by Gibson and Ming in a paper presented at the Solid FreeForm Fabrication Symposium in 1997 and entitled xe2x80x9cLow-Cost Machine Vision Monitoring of the SLS processxe2x80x9d. In this approach the concept described was to use a machine vision system (a CCD camera) to focus on the target surface of a laser sintering process and to measure gray scale color variation of the surface to calculate temperature and modify laser power to maintain consistent part quality. This approach resulted in a lower cost, simpler implementation, but was still based on an average temperature value measured by the camera system.
Thus a need exists for a more complete control scheme for laser sintering; one that measures temperatures all across the target surface and makes both global (radiant heater) and local (laser) adjustments to the heat input in order to maintain uniform temperatures.
It is an aspect of the present invention to provide a method and apparatus for fabricating objects with selective laser sintering while maintaining a more uniform temperature across the entire target surface area in the process chamber.
It is a further aspect of the present invention to provide such a method that is reliable and with acceptable cost.
It is a feature of the method and apparatus of the present invention that a laser sintering system utilizes a broad area thermal vision system, such as an infrared camera, to measure multiple temperatures across the target area and uses that temperature data as feed back to a control system.
It is another feature of the present invention that the control system both adjusts a zoned radiant heater system and the scan speed and/or laser power to control temperatures across the target area.
It is an advantage of the present invention that ideal powder layer temperatures can be estimated and used to produce three-dimensional objects with reduced distortion and curl.
It is another advantage of the present invention that the overall temperature control in the top layers of powder in the powder bed are improved.
The invention includes a method for forming a three dimensional article by laser sintering comprising the steps of: dispensing a first top layer of powder on a target area; moderating the temperature of said first top layer of powder to a predetermined goal; directing an energy beam over said target area causing said first top layer of powder to fuse powder in select locations to form an integral layer; dispensing a second top layer of powder over the fused and unfused powder of said first top layer; moderating the temperature of said second top layer of powder to a predetermined goal; directing said energy beam over said target area causing said second layer of powder to form a second integral layer bonded to said first integral layer; repeating steps (a) to (f) to form additional layers that are integrally bonded to adjacent layers so as to form a three-dimensional article, wherein the temperature moderating step comprises: using a machine vision system to image multiple temperatures of the current top layer of powder and adjust those temperatures by adjusting the radiant heat output from a zoned radiant heater located above said target area.
The invention also includes a method for forming a three dimensional article by laser sintering comprising the steps of: dispensing a first top layer of powder on a target area; moderating the temperature of the first top layer of powder to a predetermined goal; directing an energy beam over the target area causing the first top layer of powder to fuse powder in select locations to form an integral layer; dispensing a second top layer of powder over the fused and unfused powder of the first top layer; moderating the temperature of the second top layer of powder to a predetermined goal; directing the energy beam over the target area causing the second layer of powder to form a second integral layer bonded to the first integral layer; and repeating steps (a) to (f) to form additional layers that are integrally bonded to adjacent layers so as to form a three dimensional article, wherein the directing steps include the sub steps of: estimating, from known mathematical models, the desired temperatures in the region of the part being produced, then reading, from the digital output of a machine vision system the actual temperatures in the region of the part being produced, and then adjusting the energy beam power and/or scan speed during the directing step based on differences between the desired and actual temperatures to achieve desired temperatures across the target area.
The invention also includes an apparatus for producing parts from a powder comprising: a process chamber having a target area at which an additive process is performed; a means for depositing and leveling a layer of powder on the target area; a means for fusing selected portions of a layer of the powder at the target area; a machine vision system for measuring temperatures across the x-y coordinates of the target area and a radiant heater for heating the target area to control temperatures of fused and unfused powder at top surface of the target area.